A magnetoresistive head is used as a read sensor in high-recording-density magnetic recording techniques which include a hard disk as a main field, and is a part which largely influences the performance of the magnetic recording technique. It is well-known in the art that a multilayered film which is formed by stacking ferromagnetic metal layers separated by non-magnetic metal layers exhibits a large magnetoresistive effect, that is, a so-called giant magnetoresistive (GMR) effect. With respect to the magnetoresistive effect, the electric resistance changes depending on the magnetizations of two ferromagnetic layers which sandwich a non-magnetic intermediate layer therebetween and an angle made by these magnetizations. As a technique for using this giant magnetoresistive effect in a magnetoresistive element, a structure which is referred to as a spin valve has been used. The spin valve structure comprises an antiferromagnetic layer, a ferromagnetic layer, a non-magnetic intermediate layer, and a ferromagnetic layer. The magnetization of the ferromagnetic layer which is in contact with the antiferromagnetic layer is substantially pinned due to an exchange-coupling magnetic field generated on an interface between the antiferromagnetic layer and the ferromagnetic layer, and the magnetization of another ferromagnetic layer may be rotated freely by an external magnetic field, thus obtaining an output. The ferromagnetic layer whose magnetization is substantially pinned by the antiferromagnetic layer is referred to as the magnetization pinned layer, and the ferromagnetic layer whose magnetization may be rotated by the external magnetic field is referred to as the magnetization free layer.
The above-mentioned basic construction is shared in common by GMRs, specifically current-in-plane (CIP) GMRs, tunnel magnetoresistive (TMR) sensors, and current-perpendicular-to-plane (CPP) GMRs. Further, the read sensor includes a pair of magnetic shields which sandwiches the above-mentioned magnetoresistive sensor for selecting a magnetic field to be sensed from all external magnetic fields.
With respect to the magnetoresistive head, it is well-known that the magnetization of the magnetization free layer is thermally fluctuated at random due to heat, thus generating noise. The noise is referred to as mag-noise. A magnitude of the mag-noise is proportional to the inverse number of a square root of a volume of the magnetization free layer. The larger a storage capacity of a magnetic storage device, such as a hard disk, (e.g., the higher the areal density of information written in a recording medium mounted on the magnetic storage device), the smaller a size of a magnetoresistive head for reading information needs to become. Accordingly, as the areal density of information is increased, the volume of the magnetization free layer is decreased and hence, the mag-noise is increased. As a result, there arises a drawback in that a head signal to noise ratio (SNR) of the read sensor is deteriorated.